Emergency warning system with range control

ABSTRACT

A range controlled emergency vehicle warning system wherein a directional radio frequency transmission has pulses superimposed thereon at a predetermined rate are concomitantly transmitted with ultrasonic pulses at the same rate. At a receiving station in a vehicle or a traffic signal controller the radio frequency signals and the ultrasonic pulse signals are received. A gate at the receiving station responsive either to the simultaneous reception, or a preselected time phasing, of the radio frequency carried pulses and an ultrasonic pulse signal generates an enable signal to cause any radio frequency carried warning advisory signal to produce a intelligible alarm such as a visual display and/or an audible alarm in the vehicle and/or to control traffic signals during approach to a given intersection. The difference in propagation velocities of the radio frequency and ultrasonic pulses fixes a time length window to limit the range of the warning zone.

1451 Jan. 8, 1974 4] EMERGENCY WARNING SYSTEM WITH RANGE CONTROL [76] Inventor: William E. Simpkin, 4435 Fairfax,

Dallas, Tex. 75205 22 Filed: Feb. 17, 1971 21 Appl. NO.I116,007

[52] US. Cl. 340/33, 343/225 [51] Int. Cl G08g 1/01 [58] Field of Search 340/32, 33, 31 R,

340/34, 16 C, 16 R, 311; 179/1 SS, 1 VB; 325/111, 117; 343/208, 225

[56] References Cited 962,146 7/1964 Great Britain 340/3 l l Primary ExaminerWilliam C. Cooper Assistant Examiner-Randall P. Myers AttmeyRichards, Harris & Hubbard [57] ABSTRACT A range controlled emergency vehicle warning system wherein a directional radio frequency transmission has pulses superimposed thereon at a predetermined rate are concomitantly transmitted with ultrasonic pulses at the same rate. At a receiving station in a vehicle or a traffic signal controller the radio frequency signals and the ultrasonic pulse signals are received. A gate at the receiving station responsive either to the simulta- UNXTED STATES PATENTS neous reception, or a preselected time phasing, of the radio frequency carried pulses and an ultrasonic pulse i t f 24x 2 signal generates an enable signal to cause any radio 3366'925 111968 3823 i 2 340; frequency carried warning advisory signal to produce 3'532986 M970 Gelushia at 340/33 a intelligible alarm such as a visual display and/or an 3371273 2/1968 Gelushia et a]. 340/32 audible alarm in the Vehicle and/or Comm traffic 3,183,509 5 1965 Ellett 343/225 Signals during pp to a given intersection The 3,112,486 11 1963 Adler 343/208 ifference in propagation velocities of the radio fre- 3,042,868 7/1962 Kalmus et al. 325/64 quency and ultrasonic pulses fixes a time length win- FOREIGN PATENTS OR APPLICATIONS dow to limit the range of the warning zone.

1,431,735 2/1966 France 340/33 7 Claims, 1 Drawing Figure 12 42 3s 14 RADIO TuNER $2? 52 4o GENERATOR 24 f 46 I8 [6 AUDIO VOICE FREQUENCY PUSH TO TALK MIKE 22 I RECEIVER AMPITIFIER i RADIO FREQUENCY 4 i J TRANSMITTER i P i SIGNAL PULSE I SIGNAL PULSE GENERATOR DISCRIMINATOR I l 2 VARIABLE L Q 1 RANGE POT 0 -1 SYNCHRONIZER I i 36 Z 5 [62 ULTRASONIC 6O PULSE z ULTRASONIC GENERATOR PULSE AMPLIFIER xggfifig g EMERGENCY TRANSMITTER "is 123i? RECEIVER EMERGENCY WARNING SYSTEM WITH RANGE CONTROL FIELD OF THE INVENTION This invention relates to a silent controlled zone emergency or priority vehicle warning system, and more particularly to an emergency warning system utilizing the difference in atmospheric propagation velocities of radio frequency and ultrasonic signal pulses.

THE PRIOR ART Noise or operating sound systems in the drivers compartment of a vehicle often makes it impossible for the operator of a vehicle to hear the siren of approaching emergency vehicles such as ambulances, fire engines, police cars and the like. Whistles or horns of rail vehicles likewise often are masked. Further, noise pollution of sirens, horns and the like, which increase as population density and crime increase, degrade the environment.

Thus it has been proposed heretofore to transmit various radio frequency signals from emergency vehicles, the signals being received by specialized receivers located in other vehicles in order to effect an emergency warning alarm. Examples of such systems are disclosed in U.S. Pat. No. 2,963,693, issued Dec. 6, 1960, and U.S. Pat. No. 3,233,217, issued Feb. 1, 1966. Radio frequency transmitting units have been disposed upon emergency vehicles in order to control traffic lights and the like, as disclosed in such patents as U.S. Pat. No. 3,257,641, issued June 21, 1966 and U.S. Pat. No. 3,209,325, issued Sept. 28, 1965.

Problems have arisen with the use of such heretofore developed emergency warning systems, due to the lack of control over the effective range of the warning signals, and also with respect to minimizing cost and enhancing reliability. Atmospheric anomalies may cause reception of radio frequency warning signals in areas far removed from the desired location, thus creating problems by causing false warning indications in vehicles or traffic lights in remote areas.

SUMMARY OF THE INVENTION In accordance with the present invention, an emergency vehicle warning system is provided which controls the desired warning zone. A transmitter emitting the warning advisory or alarm and radio frequency pulses and ultrasonic pulses at the same rate limited in azimuth by use of a directional antenna and directional horn. A receiver activated by a predetermined time relationship between arrival or reception of the two sets of pulses provides range control. The receiver thus activated reproduces the warning advisory or alarm.

In accordance with a more specific aspect of the invention, a transmitting station has circuitry for transmitting a radio frequency signal having a sequence of pulses superimposed on the warning message oralarm transmission and circuitry for transmitting a sequence of ultrasonic pulse signals having a specific time relation to the pulses transmitted by radio frequency. A receiving station located on a vehicle to control an alarm or located along a traffic artery to control traffic signals, includes circuitry for receiving the radio frequency pulses and the ultrasonic pulses. A gate responsive at the receiving station to a predetermined time relationship between a received radio frequency pulse and a received ultrasonic pulse generates an enable signal. A switch is operable in response to the enable signal to activate a visual display and/or apply the radio frequency signals to an audio system. Transmitted warning signals are thus visually and/or audibly reproduced for the operator of the vehicle or are utilized for control of a traffic signal.

DESCRIPTION OF THE DRAWINGS For a more complete understanding of the present invention and for other objects and advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings, in which:

The FIGURE illustrates a block diagram of the preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing, an emergency warning transmitter station is identified generally by the numeral l0 and may be located for example within an emergency vehicle, a rail locomotive, or at a transmitting station located at a selected spot along a road for broadcasting appropriate prerecorded traffic or weather warning advisories. An emergency warning signal receiver is identified generally by the numeral 12 and may be located in a conventional vehicle as an element of a conventional radio, or alternatively may be disposed in a traffic light control system.

The invention will first be described with respect to an embodiment utilizing the arrival ofa short ultrasonic pulse with the time duration of a long radio frequency pulse. The transmitter station 10 includes a warning signal generator 14 comprised for instance of a stable free running oscillator. The output of generator 14 is applied to 'a movable switch arm 16 which is connected to a manually operable button switch 18. In the position illustrated, the output of generator 14 is applied to a radio frequency transmitter 20, the output of which is applied to a directional antenna 22. A microphone 24 is connected to a terminal 26. When the switch arm 16 is moved by depression of the button 18, microphone 24 is connected to transmitter 20. Transmitter 20 may then broadcast via the directional antenna 22 either the free running oscillator warning signal from the generator 14 or a radio frequency signal modulated by the voice applied to the voice microphone 24 for traffic control.

Antenna 22 is mounted upon the emergency vehicle so as to direct the radio frequency signals over a limited azimuth angle in front of the vehicle. Alternatively, the transmitter station 10 may be located along a road with the directional antenna shaped and positioned to fix the desired transmission area across the selected portion of the road.

A pulse generator 30 generates a series of predetermined intermittent pulses which are applied to the radio frequency transmitter 20 for superposition upon the RF. signal applied from the generator 14. In the preferred embodiment of the invention, the time duration or length of the pulses generated by generator 30 are controlled according to the desired range by the variable range potentiometer in a synchronizer 32. Synchronizer 32 controls the synchronization of the radio frequency pulses from generator 30 and the generation of ultrasonic pulses from a generator 34. synchronizer 32 contains a variable potentiometer for varying the length or duration of the radio frequency pulses in order to vary the distance from the transmitter over which the system is effective.

The radio frequency pulses may normally be varied in length up to 3 seconds. Ultrasonic pulses will travel approximately 0.6 mile in 3 seconds so that a 3 second R.F. pulse would fix the maximum range of approximately 06 mile. Typically, the radio frequency pulse will be of one-half second duration to fix the range at approximately 600 feet. The synchronizer potentiometer is manually operable and calibrated to enable accurate setting of the desired range.

Ultrasonic pulse generator 34 generates a sequence of ultrasonic pulses which are transmitted via a directional ultrasonic horn 36 at the same rate as the radio frequency pulses. Horn 36 is mounted upon the emergency vehicle to direct ultrasonic pulses over the same selected area in front of the vehicle as the radio frequency pulses, In the preferred embodiment, the ultrasonic pulses will be approximately 0.1 second duration transmitted approximately every 3 seconds in synchronism with the generation of the signal pulses from generator 30.

In receiver unit 12 the radio frequency signal broadcast by antenna 22 is received by a conventional antenna 38 which will be mounted upon a vehicle. Vehicles incorporating this system may have the emergency signal receiver 12 built into their radio as a subcircuit. Alternatively, a small self-contained warning system receiver may be provided for vehicles without a radio.

The antenna 38 is connected to a fixed frequency receiver 40 which is tuned to receive only the single frequency transmitted by the transmitter 20. As shown in the FIGURE, the receiver system is connected across a conventional broadcast radio tuner 42 normally installed in a vehicle. A signal diverter unit 44 is connected between the fixed frequency receiver 40 and the radio tuner 42 and the input to the audio amplifier 46. Unit 44 includes a mechanical or electronic switch 48 which selects continuity in dependence upon the logic output from an AND gate 50. The switch 48 is movable from normal contact with the output from the radio tuner 42 to the fixed frequency receiver 40 upon actuate signal from the AND gate 50.

Normally, the switch 48 contacts the radio tuner 42 such that conventional radio broadcast signals flow through the audio amplifier 46 to speaker 52 in the vehicle. Alternatively, warning lights or other visual display 45 may be actuated by an enable through the AND gate 50. The visual display may be provided in place of or in conjunction with the audio signal processed through the fixed frequency receiver 40.

The signal applied to the receiver 40 consists of the warning signal from generator 14 or voice from mike 24 and signal pulses from generator 30. The signal pulses are detected on discriminator 54 and applied to one of the inputs of gate 50 in response to each pulse generated by the generator 30.

The ultrasonic pulses transmitted by generator 34 are received by an ultrasonic transducer 60 and are applied to an ultrasonic pulse amplifier 62. The output of amplifier 62 is applied to the second input of the AND gate 50. The AND gate 50 provides three functions upon coincidence between an enable signal from discriminator 54 and a pulse from ultrasonic pulse amplifier 62. When there is coincidence, AND gate 50 triggers a time gate generator included in gate 50, for three seconds. Gate 50 actuates unit 44 to channel warning signals from receiver 40 to the audio amplifier 46. Gate 50 also turns on the audio amplifier 46 in the event it was not on at the time of warning signal reception. Gate 50 also actuates the visual display 45.

Electrical power for the receiver system is applied by operation of the ignition switch 64 of the vehicle. The system is thus energized whenever the vehicle is being operated.

In operation of the present system, the emergency unit simultaneously transmits a pulse modulated radio frequency signal from antenna 22 and ultrasonic pulses via antenna 36. Radio frequency signals received by the antenna 38 are applied to receiver 40. Discriminator 54 detects pulses superimposed upon the radio frequency signal and applies a resulting gate pulse to one input of AND gate 50. The effective range of the warning sys-' tem may be controlled by variation of the duration of the radio frequency pulses. If a vehicle carrying receiver system 12 is in close enough proximity to the emergency warning transmitter 10, an ultrasonic pulse will be received by the transducer and applied through the amplifier 62 to the second input of the AND gate 50 within the time duration of the radio frequency pulse. As a result, output of gate 50 changes state and the unit 44 is actuated by way of channel 50a to move arm 48 into contact with the output of receiver 40. The signal from receiver 40 may then be applied through switch 48 to the audio amplifier 46 and the warning signal is made audible by the speaker 52.

lfa vehicle bearing receiver 12 is located out of range of the system, ultrasonic pulses generated from the horn antenna 36 will not arrive at the transducer 60 within the time duration of the gate pulse applied to AND gate 50 from discriminator 54. In such case, gate 50 is unchanged.

An important aspect of the present invention is the fact that the ultrasonic pulses generated by the horn 36 travel at essentially the speed of sound and thus at a much slower velocity than the radio frequency electromagnetic waves transmitted by antenna 22. Thus the effective range of the system may be very accurately set by control of the duration of the pulses generated by the signal pulse generator 30. If signal pulses generated by generator 30 are shortened, the effective warning distance of the system is shortened. Conversely, if signal pulses are lengthened, the effective range of the system is lengthened because the ultrasonic pulses have a greater period of time to travel during the time gate 50 is enabled by discriminator 54. Since ultrasonic waves are not subject to erratic propagation due to atmospheric anomalies, they generally dissipate according to the inverse square relation. The range and zone of warning is therefore generally stable regardless of atmospheric changes.

If desired, switch 18 may be depressed and a voice may be transmitted from the voice mike 24 for actuating speaker 52 in order to convey particular emergency warnings, advisory or other instructions.

Although the audio amplifier 42 has been illustrated as being connected to the speaker 52, it will be understood that the output from the amplifier 46 could be utilized to control a traffic light. An example of a system for control of a traffic light in response to the reception of electrical signals is disclosed in the previously noted US. Pat. No. 3,257,641, issued June 21,

1966. In the present case, the traffic light would be placed under warning control only when within the range set by the relation between the RF. pulses and ultrasonic pulses.

In a second embodiment of the invention, selection of time phasing of the ultrasonic and radio frequency carried pulses is used to control the warning range. The circuitry of the second embodiment is generally similar to that of the first embodiment with the following exceptions. A signal pulse generator 30 generates a series of pulses which are applied through the radio frequency transmitter 20 for superposition upon the signal applied from the generator 14. The duration of the pulses generated by the signal pulse generator 30 may be about 0.] second in duration and the pulses may be generated about every 3 seconds. The variable range potentiometer and synchronizer circuit 32 provide time phasing of the pulses from generator 30 and the ultrasonic pulses from generator 34. The synchronizer 32 includes a variable potentiometer for variance of the time phasing of the pulses in order to vary the distance from the transmitter over which the system is effective.

Preferably, the ultrasonic pulses will have a duration of approximately 0.1 second and occur every three seconds in synchronism with the generation of the signal pulses from generator 30 and thus at the same rate.

The signal applied to and detected in receiver 40 consists of the warning signal from generator 14 or voice from microphone 24 with pulses from generator 30. The pulses in discriminator 54 trigger a time gate generator. The time gate generator in discriminator 54 provides an enable signal to one side of the AND gate 50.

The ultrasonic pulses transmitted by the generator 34 are received by an ultrasonic transducer 60 and are applied to amplifier 62. The output of amplifier 62 is an enable signal input to the second side of the AND gate 50. Again, AND gate 50 provides three functions upon receipt of the enable signal from the ultrasonic pulse amplifier 62 during the time that an enable signal is supplied from the time gate generator (not shown) within the signal pulse discriminator 54. When thus enabled, the AND gate provides an actuate signal for a period of 3 seconds to switch 44 and visual display 45, and further provides power to the audio amplifier 46 in the event the radio and amplifier are not previously actuated.

In operation of this embodiment, the emergency vehicle transmitters transmit a pulse modulated radio frequency signal from antenna 22 and a time phased pulsed ultrasonic signal via horn 36. The radio frequency signal is received by the antenna 38 and applied to receiver 40. Discriminator 54 detects the pulses superimposed upon the radio frequency signal and enables the AND gate 50. The gate 50 is enabled for a preset interval for reception of an ultrasonic pulse. If a vehicle carrying system 12 is in close enough proximity to transmitter 10, an ultrasonic pulse will be received by the transducer 60 and applied through the amplifier 62 to the second input of the AND gate 50 while enabled. Gate 50 actuates switch 44 to channel the output of receiver 40 to amplifier 46 and thence to speaker 52.

However, ifa vehicle bearing the emergency warning signal receiver 12 is located out of range of the system, the ultrasonic pulse generated from the horn 36 will not arrive at the transducer 60 while AND gate 50 is enabled by the time gate activated in pulse discriminator 54 by the 0.l second ratio frequency pulse. Therefore, the gate 50 output remains unchanged and the signal from receiver 40 is not applied through the switch 44.

The effective range of the system may be accurately controlled by varying the time phasing of the two sets of transmitted pulses. For example, if the ultrasonic signals lag the radio frequency signals from 0.8 to 0.1 second, the effective range will extend from about 300 feet up to about 1,100 feet. If the ultrasonic signals lead the radio frequency signals from O to l 0. 8 second, the effective range extends from about 1,200 feet to over 2,000 feet.

Again, an important aspect of the present inventionis the fact that the ultrasonic pulses generated by the horn 36 travel at essentially the velocity of sound and thus at a much slower velocity than the radio frequency electromagnetic waves transmitted from the antenna 22. Thus, the selected range may be very accurately controlled by control of the time phasing of the electromagnetic and ultrasonic pulses generated at the transmitter station.

Extreme temperature changes will tend to affect the time phasing of the synchronizer 32. Therefore, in

some installations of either embodiment of the system,

it may be necessary to utilize circuitry for compensating the time phasing for temperature variations.

When the present invention is used on locomotives for actuation of crossing signals, warning of crossing traffic and the like, the range may be automatically controlled in response to the speed of the train with appropriate inputs to the variable range potentiometer synchronizer. Thus, a fast train would be provided with a longer range than a slow or parked train.

It will thus be seen that the present invention provides an emergency warning system which is relative inexpensive and which is applicable to generally all vehicles. Emergency vehicles can thus effectively warn all other vehicle operators in a controlled zone in front of the emergency vehicle, including deaf operators. A vi sual display as well as an audio alarm may be provided within a vehicle. Additionally, emergency road advisory warning signals could be selectively broadcast over a particular area. The present system could also be used to accurately control traffic signals in a controlled range in front of an approaching emergency vehicle, even in advance of a fleeing vehicle in the-event of police chase by means of the longer range selection of the warning mode. Such signal control and emergency warning signals would terminate upon passage of the emergency vehicle. The pulses transmitted by this system are inaudible and thus avoid noise pollution characteristic of sirens. Conventional flashing lights on emergency vehicles may be retained for identification purposes.

Whereas the present invention has been described with respect to specific embodiments thereof, it will be understood that various changes and modifications will be suggested to one skilled in the art, and it is intended to encompass such changes and modifications as fall within the scope of the appended claims.

I claim:

1. A range controlled warning system comprising:

a. a first transmitter including means for transmitting general warning and advisory information on a radio frequency signal having long spaced apart pulses superimposed thereon,

b. a second transmitter for transmitting a sequence of short ultrasonic pulses at the same rate as said superir'nposed pulses,

c. a first receiver for receiving said radio frequency signal,

(1. a second receiver for receiving said ultrasonic pulses,

e. gate means responsive to the reception of one of said ultrasonic pulses within the time duration of reception of one of said superimposed pulses for generation of an enable signal,

f. an audio frequency sound channel, and

g. a switch operable in response to said enable signal to direct said radio frequency carried advisory information to said channel for audibly reproducing said warning and advisory information.

2. The warning system of claim 1 and further comprising:

discriminator means connected to said first receiver for passing only said long pulses to said gate means. 3. The warning system of claim 1 wherein said first and second receivers are located in a vehicle having a conventional radio receiver and said switch means is operable to switch said channel between said first receiver and said radio receiver.

4. The warning system of claim 1 and further comprising:

means at said transmitter for varying the time phase between said superimposed pulses and said ultrasonic pulses to vary the range of said warning system.

5. A range controlled warning system comprising:

a. a first transmitter at a sending station including means for transmitting over a limited azimuth angle along a traffic artery general warning and advisory information on a radio frequency signal having time spaced pulses superimposed thereon,

b. a second transmitter including means for transmitting ultrasonic pulses over a limited azimuth angle at the same rate as said radio frequency pulses,

c. a first receiver remote from said sending station and including mcans for rccciving said radio frequency signal,

d. a second receiver for receiving said ultrasonic pulses,

e. gate means connected to said receivers responsive to a predetermined time phase relation between said radio frequency pulses and said ultrasonic pulses for generation of an enable signal,

f. an audio frequency amplifier,

g. a switch operable in response to said enable signal to apply said radio frequency carried warning and advisory information to said amplifier,

h. speaker means for rendering audible electrical signals representative of said radio frequency carried signals, and

i. a visual display actuated by said enable signal.

6. A range controlled emergency vehicle warning system comprising:

a. a first transmitter carried by an emergency vehicle and including means for transmitting a' radio warning message and long radio frequency pulses at a predetermined rate,

b. a second transmitter on said vehicle for transmitting short ultrasonic pulses at said rate, and

c. two receivers remotely located from said transmitters in the region of the path of said vehicle to receive said message and pulses and including timing circuitry responsive to a predetermined time relationship in the arrival of a radio frequency pulse and an ultrasonic pulse at said receivers to render said warning message humanly intelligible.

7. A range controlled emergency vehicle warning system comprising:

two transmitters carried by an emergency vehicle and including selectively operable means for transmitting ahead of said vehicle at the same rate long time spaced radio frequency pulses and short ultrasonic pulses,

means for varying the time phase between said radio frequency pulses and said ultrasonic pulses, and

two receivers remotely located from said transmitters and including circuitry responsive to a predetermined time phase relationship between received radio frequency pulses and ultrasonic pulses to pro- 

1. A range controlled warning system comprising: a. a first transmitter including means for transmitting general warning and advisory information on a radio frequency signal having long spaced apart pulses superimposed thereon, b. a second transmitter for transmitting a sequence of short ultrasonic pulses at the same rate as said superimposed pulses, c. a first receiver for receiving said radio frequency signal, d. a second receiver for receiving said ultrasonic pulses, e. gate means responsive to the reception of one of said ultrasonic pulses within the time duration of reception of one of said superimposed pulses for generation of an enable signal, f. an audio frequency sound channel, and g. a switch operable in response to said enable signal to direct said radio frequency carried advisory information to said channel for audibly reproducing said warning and advisory information.
 2. The warning system of claim 1 and further comprising: discriminator means connected to said first receiver for passing only said long pulses to said gate means.
 3. The warning system of claim 1 wherein said first and second receivers are located in a vehicle having a conventional radio receiver and said switch means is operable to switch said channel between said first receiver and said radio receiver.
 4. The warning system of claim 1 and further comprising: means at said transmitter for varying the time phase between said superimposed pulses and said ultrasonic pulses to vary the range of said warning system.
 5. A range controlled warning system comprising: a. a first transmitter at a sending station including means for transmitting over a limited azimuth angle along a traffic artery general warning and advisory information on a radio frequency signal having time spaced pulses superimposed thereon, b. a second transmitter including means for transmitting ultrasonic pulses over a limited azimuth angle at the same rate as said radio frequency pUlses, c. a first receiver remote from said sending station and including means for receiving said radio frequency signal, d. a second receiver for receiving said ultrasonic pulses, e. gate means connected to said receivers responsive to a predetermined time phase relation between said radio frequency pulses and said ultrasonic pulses for generation of an enable signal, f. an audio frequency amplifier, g. a switch operable in response to said enable signal to apply said radio frequency carried warning and advisory information to said amplifier, h. speaker means for rendering audible electrical signals representative of said radio frequency carried signals, and i. a visual display actuated by said enable signal.
 6. A range controlled emergency vehicle warning system comprising: a. a first transmitter carried by an emergency vehicle and including means for transmitting a radio warning message and long radio frequency pulses at a predetermined rate, b. a second transmitter on said vehicle for transmitting short ultrasonic pulses at said rate, and c. two receivers remotely located from said transmitters in the region of the path of said vehicle to receive said message and pulses and including timing circuitry responsive to a predetermined time relationship in the arrival of a radio frequency pulse and an ultrasonic pulse at said receivers to render said warning message humanly intelligible.
 7. A range controlled emergency vehicle warning system comprising: two transmitters carried by an emergency vehicle and including selectively operable means for transmitting ahead of said vehicle at the same rate long time spaced radio frequency pulses and short ultrasonic pulses, means for varying the time phase between said radio frequency pulses and said ultrasonic pulses, and two receivers remotely located from said transmitters and including circuitry responsive to a predetermined time phase relationship between received radio frequency pulses and ultrasonic pulses to produce an emergency alarm. 